1. Field of the Invention
The present invention relates to an optical fiber for attenuating optical signals, which is used in an optical attenuator, optical terminal device or the like.
2. Description of the Related Art
In recent years, optical fiber communication networks have made rapid progress. In the optical fiber communication network, a variety of optical devices are used.
As one of the optical devices, there is an optical fixed attenuator, which is used to positively attenuate optical signals and adjust the light power level in the optical fiber communication network into proper scope.
Further, as another of the optical devices, there is an optical terminal device, which is used to reflect the input light in the output side and make the reflected light return, then attenuate the light having returned, for removing the influence of unnecessary optical signal.
In these optical devices, generally, optical fiber for attenuating optical signals is used. The optical fiber for attenuating optical signals has a core formed from quartz glass, which has a higher refractive index by doping with Ge (germanium). In the core, Co (cobalt) is contained as a dopant (doped material) to positively attenuate optical signal.
Moreover, in a WDM (Wavelength Division Multiplexer) network, the optical fixed attenuator or the optical terminal device is needed. In the WDM network system, there are many kinds of wavelengths with respect to various transmitted light signals, and the attenuation amount of the optical fixed attenuator or the optical terminal device must correspond to any one of the wavelengths.
Therefor, the optical fiber for attenuating optical signals used in the optical fixed attenuator or the optical terminal device, it is desired to obtain the same attenuation amount, not only corresponding to the optical signal having the wavelength of 1310 nm or 1550 nm, but also corresponding to the optical signals having wider wavelength scopes.
Conventionally, in order to obtain such fiber for attenuating optical signal, whose light attenuation amount is constant, there is a method described in Japanese Patent publication 8-136736. This method uses a characteristic that mode field diameter (MFD) depends upon the wavelength of the transmitted optical signal, and adds dopant to a narrow area in core, then provides an appropriate construction.
However, in the conventional art, there are some problems that need to be solved. That is, Co is a dopant used in the optical fiber for attenuating optical signal as usual. As a characteristic of the Co, the longer the wavelength of optical signal, the greater the Co absorbs the optical signal. Because the dopant absorbs the optical signal, the optical signal is attenuated. By using the Co having such characteristic and adding it to core, the optical fiber for attenuating optical signal is formed, which can obtain stable attenuation amount with respect to various optical signals having various wavelengths. Thus, as stated above, it is possible to use a characteristic depending upon the wavelength of optical signal transmitted by mode field diameter (MFD), to add dopant to the narrow area in core, and to provide an appropriate construction.
However, when using such a method, which adds dopant to the narrow area in core, the passage of the optical signal becomes narrow. Therefore, in order to obtain the desired attenuation amount, it is necessary to improve the concentration of the Co. In the case, when using a method called sinking method, it is difficult to add Co at a higher concentration.